Wall thickness and the presence of defects such as cracks are important factors in determining the fitness-for-service of structures such as above- and below-ground pipes and tanks, including bulk material and weldments. When a pipe is in operation, it can be subject to corrosion and/or erosion due to the content, flow and/or environmental conditions inside or outside of the pipe. Cracks can form and propagate due to the presence of manufacturing defects, creep, thermal cycling, fatigue and environmental conditions causing defects such as high-temperature hydrogen attack (HTHA), stress-corrosion cracking, etc. Corrosion and/or erosion results in the reduction in wall thickness, which can reach a point at which operating conditions becomes unsafe, considering that the pipe can be pressurized and may contain hazardous or flammable materials. Likewise formation and propagation of cracks, in welds for instance, can cause similar unsafe conditions. A failure may cause catastrophic consequences such as loss of life and environmental damage, leaking pipes, in addition to the loss of the use of the asset, and any corresponding costs associated with repair, loss of capacity and revenue loss.
Ultrasonic non-destructive evaluation (NDE) techniques are commonly used for evaluating the integrity of industrial components. In the case of measuring wall thickness reduction due to erosion/corrosion, the traditional process involves using a portable handheld instrument and ultrasonic transducer (probe) to measure the wall thickness. The instrument excites the probe via an electrical pulse, and the probe, in turn, generates an ultrasonic pulse which is transmitted through the structure. The probe also receives an echo of the ultrasonic pulse from the structure, and converts the pulse back into an electrical signal. The ultrasonic pulses that are transmitted into and received from a structure are used to determine the relative position of the surfaces (i.e. thickness) of the structure wall. More specifically, by knowing the travel time of the ultrasonic pulse from the outer wall to the inner wall and back (ΔT) and acoustic velocity (V) of the ultrasonic pulse through the material of the structure (through calibration or just initialization), a wall thickness (d) can be calculated—i.e. d=ΔT*V/2. There are many variants of these two basic descriptions of ultrasonic thickness gauging and flaw detection that are known to skilled practitioners of ultrasonic nondestructive evaluation.
These approaches require an operator to manually position a probe on the wall of the asset to take a reading. Not only does this necessitate the operator manually taking each reading, but also the measurement location must be accessible, which can be challenging and costly. For example buried pipelines require excavation to access, insulated pipe requires costly removal of the insulation, offshore assets require helicopter or boat access, and elevated vessels requiring scaffolding or crane access. While the measurement is relatively simple, the cost of access (scaffolding, excavation, insulation removal, etc) is often much higher than the cost of measurement. Moreover, the operator is often subjected to hazardous conditions while taking the readings. Furthermore, to obtain trending data with thickness resolution of 0.001″ or better requires that the transducer be placed in the same exact location for consistent readings at regular time intervals. This is difficult and often impractical especially when the data-capture rate needs to be frequent. Variations in operator and/or equipment tend to skew the quality and integrity of the measurement data.
One approach for avoiding some of the aforementioned problems is to use installed sensors/systems for asset-condition or asset-integrity measurement. The sensors are permanently or semi-permanently installed on the asset and can take advantage of features such as wireless data transmission to avoid costly wiring installations. Automated systems require no operator to be in the vicinity of the asset and can stream data to a control room or to an operator's desk. Current state of the art devices/systems have been shown to be useful and commercially successful for permanently monitoring structures using ultrasound.
While current state of the art devices are useful and valued for corrosion monitoring, Applicants have identified two major shortcomings in that current devices are high cost and require large amounts of power to operate. The large amount of power requires large, lithium ion batteries (often D size cells) to meet required battery life requirements of 5-10 years with a 1/day measurement cycle. These large lithium ion batteries negatively impact cost and have safety and shipping issues due to the large volume of lithium. Furthermore, the volume of the batteries limits the amount of miniaturization that can be accomplished.
There are several factors that drive the power and cost of current state of the art systems. The first involves the relatively high frequencies used in ultrasonic NDT. Typical thickness gauging is carried out between 1 and 10 MHz to achieve the proper balance between penetration power, beam characteristics and near-surface resolution. According to Nyquist's theorem, the digital sampling of signals of frequency F, must be at a rate greater than 2F to properly represent the signal. As the signals used in NDT are typically pulses, the bandwidth of those signals and the desire for accurate timing measurements drives the requirement of even higher sampling rates, in the range of 5 to 10× the center frequency of the transducer. Sampling rates of 40 to 100 MSPS are commonly used. This requirement drives the system architecture to be somewhat complex, requiring a high-speed digitizer, and usually FPGA (field programmable gate array) to handle the high-speed signals. The additional components increase power consumption during signal acquisition, require more complicated power supply architectures to handle the multiple voltages necessary for the circuit, and increases circuit board size and component cost.
Applicants have also identified a second factor, particularly for installed sensor systems with wired transducers is the cost of long cabling, connectors, installation and signal degradation. Cabling with suitable electrical characteristics and appropriate for the harsh environments can cost several dollars per foot. Coaxial connectors are also expensive from both a part and assembly cost. Cable and connector costs can easily exceed $50-100. The high-frequency signals that connect the probe to its ultrasonic instrument, degrade considerable over distances as short as 8 meters. Labor costs for the type of industrial wiring and conduits at Oil & Gas or Power Plant sites often renders wiring too costly to be practical especially for semi-permanent installations.
A third factor is the choice of using mesh network topologies. In a mesh network, devices are not only responsible for handling their own data, but are also responsible for repeating messages for neighboring devices. As such, devices on a mesh network must always be connected to the network to be ready to receive, transmit, or repeat a message. Current mesh network protocols based on the 802.15.4 standard are often deployed on the unlicensed 2.4 GHz Industrial, Scientific and Medical (ISM) band. Range of such networks are usually less than 200 meters, often requiring extra repeater nodes to achieve connectivity of devices over a large facility. Sometimes one or more repeater nodes are required for each operating sensor to achieve the necessary wireless coverage.
To address these shortcomings, Applicants have designed a novel device for implementing a permanently installed thickness gauge that is wireless, low power, miniature and only requires single small battery to run for significant amounts of time (e.g., 5-10 years).